In the manufacturing of integrated circuits, the sizes of integrated circuit devices are scaled down increasingly. For example, Complementary Metal-Oxide-Semiconductor (CMOS) Image Sensor (CIS) chips have increasingly smaller pixel sizes. Accordingly, the requirement in the DC and noise performance of the CIS chips becomes increasingly stricter. Conventional integrated circuit formation processes thus cannot meet the strict requirements of the CIS chips. For example, Shallow Trench Isolation (STI) regions were used to isolate devices. In the formation of the STI regions, the silicon substrate, in which the STI regions are formed, suffers from the damage caused by the formation of the STI regions. As a result, charges such as electrons are trapped at the interfaces between the STI regions and the silicon substrate. These charges cause background noise in the signals of the CIS chips.
To solve these problems, Device Isolation (DI) regions are formed to replace the conventional Shallow Trench Isolation (STI) regions to isolate devices. With the formation of the DI regions using implantation rather than STI regions, the damage to the surface of silicon substrate, which damage is caused by the formation of the STI regions, is eliminated.
The DI regions are formed by implanting an impurity to portions of a substrate, which portions surround the active regions of the integrate circuit devices that are to be isolated. The implanted impurity has the same conductivity type as the well regions in which the devices are formed. It is, however, difficult to control the accuracy in the overlay of the components of the integrated circuit devices with the DI regions. For example, the accuracy in the overlay between the DI regions and the overlying gate electrodes are difficult to control. In the situations in which the gate electrodes are misaligned to the DI regions, gate electrodes may not be able to fully separate the channel between the source and drain regions of the MOS devices formed at the DI regions. Leakage currents may occur between the source and drain regions.